What goes into creating those vibrant reds, oranges and yellows that takeover trees in the fall? The latest edition of Science Behind the Forecast lets us know.
Bill Burton: It's time for us to take a look at the Science Behind the Forecast as I am joined by WAVE 3 meteorologist Tawana Andrew. Good morning. Tawana.
Tawana Andrew: Good morning. We're talking about one of the most beautiful parts of fall today.
BB: Always been my favorite part, the changing of the leaves, the red, the yellow, the orange, the all sorts of different shades. Fall colors, who doesn't love that? What makes the leaves look so spectacular for such a relatively short time in the fall?
TA: So first thing we need to think about is the amount of daylight that we get at this time of the year. So Earth's daylight hours start to decrease on the summer solstice, which is usually June 20, 21st or 22nd. And this continues all the way through the winter solstice, which is usually December, 21 or 22nd. So when you get to the point where the day is the shortest and the night is the longest. Now a trees' leaves will change color a the total amount of daylight hours start to decrease, and when you get into this time frame, the amount of photosynthesis happening within a plant starts to slow down. Photosynthesis. Of course, we learned this in have to be like elementary school and middle school, right?
BB: Good to have a refresher.
TA: Super important, because it is why these plants produce chlorophyll, and how they produce chlorophyll, it turns sunlight into much needed glucose that the plants need to survive. Now this chlorophyll basically saturates the leaf, and that is why they look green. Now if you start to get less of that photosynthesis happening because you're getting less of that sunlight, that means you're seeing less chlorophyll and more of the leaf's true colors. This is where you get the reds, the yellows, the oranges shining through now the amount of red and the time that the leaf's color is displayed is directly dependent on the moisture and the temperatures present before and during the time that chlorophyll begins to decrease. So we really need to pay attention to moisture if we really want those vibrant reds. Now, the best fall foliage displays will come after warm, sunny days and cool but above freezing nights. If you think about the past couple of weeks we've gotten that here in our area, so it is helping us a lot. Now, the sunshine on those warm, sunny days helps to promote the creation of sugars in the leaves, and with the cooler nights. What that does, along with the narrowing leaf things that typically happens during the fall, it guarantees that the sugars that are produced during the daytime hours are trapped in the leaf. So that is why, if we have warmer fall temperatures, that can delay or shorten the peak fall foliage time frame. And of course, you know me, I'm going to dive into the numbers. So on average in Louisville, we've seen about a three degree increase in fall night time temperatures between 1970 and 2023. And this is important because here's the fun fact that you're going to be telling everybody, research has found that you need a nine degree Fahrenheit to 12 degree Fahrenheit difference between day and night time temperatures to significantly enhance fall color. So that nine to 12 degree Fahrenheit difference is very important.
